The development and progression of osteoarthrosis, rheumatoid arthritis and ankylosing spondylitis are being determined by means of clinical, radiographic and serological examinations carried out prospectively at two-yearly intervals among adults of the Gila River Indian Community (Pima Indians) in Arizona, in conjunction with epidemiological studies of diabetes in the same community. The purpose of this investigation is to ascertain the determinants of these diseases in the population, and to identify factors which alter the natural history or progression of the diseases. The relationship of host factors, including various gene markers, such as HLA and Gm type, and age, sex, together with various potential environmental determinants, such as obesity, and evidence of exposure to infectious agents, to the development of these diseases will be determined prospectively. Longitudinal data have now been collected over a 20 year period in the population, and represent a unique data-set for epidemiological studies of arthritis.